The assessment of primary silicon constituents on the surface of silicon-containing aluminium alloys is as a rule effected light-optically with a microscope. However, a requirement for such a test is that the surface to be tested is as smooth as a mirror so as to bring about the required good reflection of light. Such a surface is in practice obtained by grinding and polishing the workpiece surface to be looked at. A microscopic evaluation is thus only possible if either the required dead-smooth surface is given from the outset or such a surface is specially produced for the performance of the test.
There are cases where the test is to be effected on the surface of a finished workpiece and where the surface of this workpiece is neither fine-polished nor allowed to be fine-polished. An example are the cylinder bores of internal-combustion-engine blocks consisting of a silicon-containing aluminium alloy. For the attainment of a good running or sliding behaviour between the piston and the cylinder wall, the cylinder surfaces are honed in a very specific manner. If one wants to test the primary silicon distribution over, for example, the entire height of the cylinder bore on such a surface, which must not be charged for test purposes, a microscopic inspection is ruled out because without receiving additional treatment the surface is unsuitable for such an inspection. A non-destructive test is thus not possible with the use of a microscope.
Another possibility of testing the distribution of primary silicon crystals on the surface of an aluminium alloy is the so-called imprinting method. In this method, foils are pressed on the surface to be tested and the impressions in the foil are subsequently evaluated. However, this method only works if the silicon particles project from the surface to be tested, i.e. if differences in height exist between the silicon to be determined and the surrounding aluminium matrix. If this is not the case, such an imprinting method by means of a foil fails.
Another method for the detection of silicon in the surface is the measurement of the surface roughness. But this method also necessitates a projection of the silicon particles from the remaining aluminium matrix.
In summary, it can be said that the hitherto known measuring methods allow a non-destructive test only in exceptional cases, namely when the surface formations required for the various measuring methods happen to be provided.